1. Field of the Invention
The present invention relates to a rotary electrical device driven by a three-phase power supply whose stator and rotor cores are provided with bonded permanent magnet strips, producing a high torque at slow operating speeds for use in electrical motors such as vernier motors.
2. Description of the Related Art
FIG. 1 is a schematic cross sectional view of a conventional vernier motor. A stator core 11 is provided with Z.sub.1 slots 24 for installing three-phase windings to produce rotational magnetic fields of p magnetic pole pairs, and is also provided with permanent magnets 15 which are bonded to the inner surface of each slot 24. It follows that there are Z.sub.1 permanent magnets provided for the stator core 11. For the rotor core 10, there are Z.sub.2 permanent magnets 16 provided equidistantly along its outer peripheral surface. The number of permanent magnets Z.sub.2 on the rotor core and the number of permanent magnets Z.sub.1 on the stator core are related by the following expression: EQU Z.sub.2 =Z.sub.1.+-.p
The permanent magnets on the stator core 11 and those on the rotor core 15 are arranged so that the magnetic poles are identically oriented in the radial direction. That is, if the polarities of the permanent magnets 15 on the stator core 11 are such that the outer side is the N-pole and the inner side is the S-pole, then the permanent magnets 16 on the rotor core 10 are arranged so that the polarities are also the N-pole on the outer side and the S-pole on the inner side.
Such a vernier motor is operated by rotating magnetic fields produced by the three-phase windings in the slots on the stator core, and exhibits a characteristic ability to produce a high output torque at low speeds, because of the effects of the permanent magnets having attracting poles across a clearance gap between the stator and rotor cores. The rotational speed .omega.v of such a vernier motor is given by: EQU .omega.v=.omega./Z.sub.2
where .omega. is the angular speed of the rotating magnetic fields produced by the alternating current supplied to the three-phase windings. Therefore, the vernier motor can be operated at a rotational speed that is 1/Z.sub.2 lower than the angular speed .omega. generated by normal rotating magnetic fields. Similarly, the rotational torque Tv is given by: EQU Tv=(Z.sub.2 /p).times..tau.
where .tau. is the normal torque produced in a permanent magnet type synchronous motor, so that an output torque produced is (Z.sub.2 /p) times greater than that produced by a normal permanent magnet type synchronous motor.
Such vernier motors are, therefore, ideally suited to applications such as wafer polishing apparatus that requires a high turning power at slow rotational speeds. In a polishing apparatus to produce a flat and mirror polished surface on a semiconductor wafer, the top ring holding the wafer is pressed against the polishing surface (cloth or fixed abrasive materials) mounted on a turntable at a given pressure, and both are rotated at low speeds with a polishing solution (slurry or pure water etc.) at the contact interface of the wafer and the polishing surface, until polishing is completed. Such a polishing apparatus is operated typically at 15 r.p.m., and the motor is required to produce a high output torque to overcome a large frictional resistance generated at the contact interface of the wafer and the polishing surface. Normal induction motors do not easily exhibit the characteristics necessary for such low speed, high torque operations, so that vernier motors are much more suitable for such applications.
However, the vernier motor shown in FIG. 1 presents the following operational problems. The first problem is that, because the permanent magnets 15 are placed at the inner opening of the slot 24, magnetic resistance is increased in a slot section, and correspondingly, the total available magnetic flux is reduced. This has an effect of reducing the torque that can be generated by the motor.
The second problem is related to the design of the motor. Because the permanent magnets 15 are to be placed at the opening of each slot 24 of the stator core, the magnets must be bonded after the conductive wires of the windings are installed in the stator slots and fixated with varnish. In other words, after the wires are installed in the slots, varnish is used to enhance electrical insulation as well as the fixate the wires inside the slots. However, varnish applied to the permanent magnets 15 interferes with a bonding operation of the permanent magnets 15 to the inside surfaces of the stator core 11 with an adhesive agent, so that it becomes difficult to produce secure bonding of the magnets on the stator core. Even more important is a loss of the distinguishing feature, which is the narrow clearance gap between the stator core 11 and the rotor core 10, because of the unavoidable variations that occur in the varnished conditions at the opening section. Such variations in the gap dimension is a primary cause of variability in the performance of vernier motors produced by the conventional assembly method.
Remedial steps that may be taken include masking of the bonding surfaces of the stator core beforehand to avoid application of varnish thereto, or removal of varnish from the bonding surfaces before bonding. These steps are very cumbersome and labor-intensive, and require careful process monitoring because any residual varnish can affect the gap size and consequently the motor performance.